Postage On Philatelic Ship Covers

The three covers below all have 1.5¢ Hardings. They all have card stock inside. The first and third ones are sealed and the second one has the flap tucked in. Is the postage correct on all three?

As a follow up, I am ignorant of the postal regulations during that period. My 2014 Scott Specialized only has Domestic Letter and Air Mail rates and an internet search on third class rates in the 1930s was not productive.

Yes, the rate for 3rd class was 1.5 cents at this time for things like holiday and birthday cards, etc. Very minimal added writing like signatures. Sent unsealed.

Yours would have been unsealed when they were mailed, or they would have been charged 3 cents as a first class letter. Sealed now means nothing. Many collectors consider "sealed" to look more natural and licked the flap when they arrived.

For rates, obtain a copy of Beecher & Wawrukiewicz's "U.S. Domestic Postal Rates, 1872-2011", 3rd ed. from the APS.

Thanks, John. I thought that perhaps a collector had sealed the two; one would think tucking the flap instead would preserve the clarity of the rate, but not an issue for the collector.

You may be overthinking this. It is impossible to tell today whether the flaps on the two now-sealed covers were tucked or un-sealed when they were cancelled.